libgimp: API with new object classes are since 3.0.

Even when the function names may have stayed the same in most cases, the
API has changed. The "Since:" tag must therefore be bumped.

Also adding docs for gimp_drawable_get_sub_thumbnail_data() which had
none.

libgimp/gimpchannel.c

@@ -66,6 +66,8 @@ gimp_channel_init (GimpChannel *channel)
  *
  * Returns: (transfer none): The newly created channel.
  *          The object belongs to libgimp and you should not free it.
+ *
+ * Since: 3.0
  */
 GimpChannel *
 gimp_channel_new (GimpImage     *image,